The invention relates to a method for preparing the surface of a semiconductor substrate, in particular to a method providing an oxide-free semiconductor substrate surface.
For certain applications, it is necessary to provide semiconductor substrates from which the natural oxide layer has been removed. One such situation, for example, appears in the so called. Direct Silicon Bonding (DSB) technology during which typically a silicon (100) surface of a first substrate is directly bonded to a silicon (110) surface of a second substrate without intervening oxide layer. To be able to successfully bond the two substrates, it is important to remove all possible sources of oxygen or oxide at the bonding interface to prevent the creation of oxide or oxygen precipitates which would delimit the quality of the obtained DSB substrate. For example, the presence of such precipitates or oxides at the bonding surface would be responsible for low bonding energies, leading to insufficient adhesion between the two bonded substrates.
The same kind of problem occurs during the process of forming a strained silicon layer. To produce a strained silicon layer, one typically starts from a single crystal silicon support substrate on which a relaxed silicon germanium (SiGe) layer is produced via a buffer layer. This buffer layer is an intermediate layer between two crystallographic structures with different lattice parameters, having in the region of one interface, a lattice parameter substantially identical to that of the first structure (the silicon substrate) and in the region of its other interface, a lattice parameter substantially identical to that of the second structure (the relaxed SiGe layer). Typically, the buffer layer is a SiGe layer in which the germanium content progressively or in steps increases through the thickness. Then, on the relaxed buffer layer, a thin second layer is grown (typically with a thickness of about 200 Å) which adapts its lattice constants to the one of the underlying relaxed SiGe layer and becomes a strained Silicon layer (sSi). Like in the previous situation, the crystalline quality of the strained silicon layer is negatively influenced by the presence of oxygen or oxides on the surface of the relaxed SiGe layer.
In the prior art, the solution to the above-described problem consists in an HF treatment, which leads to a hydrophobic surface state, which avoids a native oxide layer to grow. Nevertheless, it was observed that, even with the HF treatment, only a low bonding energy was achieved in DSB applications and that defects were still present in the strained silicon applications, like for example, the so-called “watermarks”.